A via is a vertical microscopic tunnel that penetrates selected inter-metal dielectric layers (IMDs) on the surface of a semi-conductor wafer and is filled with a conductive filler to provide an electrical flow path. Typically, the via is connected to a conductive layer at both its ends.
Vias are etched into a dielectric layer by exposing selected areas on the surface of the dielectric layer to etching processes. Where vias are not to be formed, the surface is covered with an etch-resistant material during etching, which is removed after the vias are etched. How deeply a via is etched into one or more dielectric layers depends on factors such as etch method, etch rate and etch time.
When the etch time is insufficient, the via does not penetrate sufficiently through the dielectric layer, or layers, into contact with an underlying conductive layer or device element. Therefore, vias are sometimes slightly over-etched to ensure that the vias are cleared of all dielectric material.
Several methods may be used to monitor sufficient via depth, such as profilometry, X-ray Scanning Electron Microscopy (X-SEM), Atomic Force Microscopy (AFM) and via resistance measurement. However, profilometry has limited accuracy in profiling surface features having dimensions as small as that of a via. SEM techniques are sample destructive and slow, since the IMD has to be cut to reveal the via cross-section, and are therefore unsuitable as quick means of quality control. AFM requires tedious changing of cantilever tips and is too troublesome to be incorporated into a manufacturing process for quality control. Via-chain resistance measurement is the most commonly used quick-detection technique for monitoring good via connections, which would mean that the vias are not under-etched. However, if the via connections are bad, i.e. have high resistance, via-chain resistance measurement cannot distinguish whether the bad connection is due to under-etching, or via-misalignment leading to non-contact with the underlying conductive layer. Furthermore, via-chain resistance measurement cannot tell us how much via depth is short in the event of under-etching.
It is, therefore, desirable to provide a method that is sensitive and quick in response for selectively detecting under-etching or via misalignment. It is preferable if the method is also able to indicate by how much the via depth is short of reaching the target depth.